Olfaction is the primary sense used by most animals to detect environmental cues and generate adaptive behaviors, including obtaining food and avoiding predators. A large family of olfactory receptors (ORs) support the detection and discrimination of a wide variety of odorants in the environment. In mice, these ORs are encoded by a gene family containing more than one thousand members. Each mature olfactory sensory neuron (OSN) stochastically expresses one OR, which defines the tuning properties of individual OSNs. While each OR detects a unique subset of odorants, all ORs signal through common second messenger pathways, suggesting that OSN response properties are exclusively determined by which OR is expressed by each OSN. However, OSNs expressing different ORs also differentially express a handful of axon guidance molecules during development, suggesting that OSNs might express genes (in addition to the OR) that distinguish different OSN populations. However, outside of these developmental differences (and the small number of adult differentially-expressed genes known to be associated with dorsal or ventral identity), it is unclear if there are more pervasive difference in gene expression that distinguish mature OSNs and that might influence the functional responses of OSNs to odorants. To address this question, we recently performed single cell RNA- sequencing of more than 50,000 mature OSNs. We observed extensive differences in gene expression across OSNs expressing different ORs, which affected the levels of more than one thousand genes. Strikingly, we found that all the OSNs expressing a given OR were transcriptionally similar to each other and distinguishable from cells expressing other ORs, allowing us to accurately predict which OR (out of 1,000) a given OSN expressed based upon each cell's particular pattern of gene expression. Furthermore, preliminary data suggests that neural activity downstream of each receptor may elicit systematic changes in gene expression. Combined, these results suggest that each OR is associated with a stereotyped OR-specific gene expression program, and that this gene expression program reflects (at least in part) the activity of each OR. We aim to study the establishment, maintenance, and functional consequences of this diversity in gene expression patterns across OSNs. In Aim 1, we will investigate how ORs influence their associated gene expression programs by manipulating OR activity and ectopically expressing ORs at different stages of OSN differentiation. In Aim 2, we will take advantage our ability to use gene expression as a surrogate for neural activity to query how odorants with neutral and innate behavioral relevance are detected in vivo by the complete array of ORs. Taken together, my proposed research will provide insight into how ORs shape gene expression patterns in OSNs and how these patterns influence functional responses to odorants. By surveying gene expression differences and changes across the entire repertoire of ORs, these experiments will reveal core principles of the relationships between ORs, neuronal activity, and sensory processing of odor stimuli.